Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Beezerboy on June 24, 2020, 04:28:22 pm

Title: hum in my Plexi
Post by: Beezerboy on June 24, 2020, 04:28:22 pm

I built the Hoffman 6V6 Plexi pretty much by the plans other than I made my own chassis out of aluminum (aircraft 2024). the amp works & sounds great but has a noticeable hum (especially with single coils) when a guitar is plugged in. with no cord in the jack it almost totally silent. heres the odd part... if I touch the chassis 90% of the hum goes away. it does not matter if I am also touching the guitar. I've been playing it for a couple years and some venues its not hardly noticeable (but always there a little bit). I recently isolated the input and speaker jacks from the chassis and no change. I've tried different cords... some make it worse. seems like I've rolled V1 a time or two. I used shielded wire where the plan specified. I did some searches and nothing like this came up. any thoughts?

Title: Re: hum in my Plexi
Post by: shooter on June 24, 2020, 04:36:40 pm

Quote

if I touch the chassis 90% of the hum goes away

without anything plugged in at input?
without nothing plugged in does the hum get worse when knobs are increased?
knowing 60hz 120hz or some other "interference" would help

essentially you're "shorting out" the antenna (the amp)
broken shield is my 1st guess
2nd, the amp bottom isn't "grounded" well, maybe add more aluminum tape

more guesses when I start playing with crayons  :icon_biggrin:

Title: Re: hum in my Plexi
Post by: Beezerboy on June 24, 2020, 10:45:48 pm

quiet when there is nothing plugged in. only normal hiss levels which are quite small

all the tubes have metal shields

I'll try some more shielding

Title: Re: hum in my Plexi
Post by: Latole on June 25, 2020, 03:56:10 am

"... if I touch the chassis 90% of the hum goes away."

IMO chassis is not grounded to wall outlet.

Title: Re: hum in my Plexi
Post by: shooter on June 25, 2020, 07:55:31 am

Quote

quiet when there is nothing plugged in.

so you're "inducing" the noise from "outside", your guitar and cable.
I'd go back to your grounded input jack
I'd verify my amp grounds.

my last amp did something similar, but I knew it was a bad guitar cable and sketchy guitar wiring, fixed those, fixed my hum

Title: Re: hum in my Plexi
Post by: dude on June 25, 2020, 09:35:43 am

Run a ground wire from input jack's ground to where your pre-amp ground is grounded, should be right next to the the input jack.

Title: Re: hum in my Plexi
Post by: Beezerboy on June 25, 2020, 06:43:48 pm

the input jack grounds on the pre-amp bus along with all the pots. one long bus wire connects the lot. right now the bus grounds to a lug through the chassis near the input jack.   is there a better place to ground the pre-amp bus? I connected a jumper wire on the jack and tried jumping it to various points on the chassis.... made no difference but wasn't soldered, etc

the transformer center tap and ground for the power supply filter caps all tie to one of the tranny's mounting  screws, along with the green ground wire from the power plug

the hum mostly goes away when I touch the chassis. doesn't matter if I touch the guitar or not. crazy

Title: Re: hum in my Plexi
Post by: PRR on June 25, 2020, 06:52:20 pm

> hum mostly goes away when I touch the chassis. doesn't matter if I touch the guitar or not.

With the guitar in your hand? Or across the room? When your mate touches the chassis?

Whatever is going on is strange, and we can't see it from where we are. You can experiment and report. Maybe shake-loose a good thought from someone.

Title: Re: hum in my Plexi
Post by: Latole on June 26, 2020, 02:52:09 am

Amp power cord look right. Is wall outlet ground is OK ?

Can you post good pictures of the amp wiring. It may help.